This research addresses 1) age-related declines in the intrinsic capacity of the brain to exhibit plastic change of a type thought to be involved in memory formation, 2) effects of physical exercise upon the morphology of the aging brain, and 3) effects of mental activity the morphology of the brain. A central aspect of the approach is the increasingly strong evidence that the formation and/or stabilization of synaptic connections between nerve cells is involved in memory and related forms of brain information storage. In the aging human and animal brain, there is evidence that the declines apparent in memory are paralleled by the loss of synapses, a reduced ability to form new synapses, or both. The underlying causes of these changes in the aging brain are unknown, and determining what they are is the chief goal of this research project. The causes could be intrinsic to the brain or could involve physiological-metabolic support systems such as the cardiovascular system. The brain or peripheral systems could, as recent work suggests, be modulated by behavioral variables such as physical exercise or mental activity. The hypotheses to be tested involve the following questions: 1) does the aging brain have a diminished ability to generate new synaptic connections between neurons? 2) Does physical exercise affect number of or inducibility of synaptic connections, perhaps through general effects upon vascular or other metabolic support ? 3) Does physical exercise affect brain vasculature or metabolism in general? 4) Does motor learning (of an "acrobatic" task) affect the number of synaptic connections in brain regions likely to be heavy involved in performance of the task? 5) Does more general "mental exercise" arising from living in a complex environment affect brain metabolic or connectivity measures? And 6) Does additional physical exercise potentiate the effects of living in a complex environment? The hypotheses are to be tested using qantitative morphological methodology, combining light and electron microscopy to assess numbers of synapses per neuron, synaptic and neuronal density, vascular morphology, and mitochondrial volume fraction. These questions are critical to an understanding of the factors involved in the brain aging process and associated declines in learning, memory, and other aspects of mental performance and well-being.